(Scanned from Science News, 5/28/94 issue, used without permission, but probably within th
(Scanned from Science News, 5/28/94 issue, used without
permission, but probably within the copyright law definition
of "fair use." Any text errors are the result of the scanning
"For scientists interested in how life came about, the
chicken-and-egg controversy boils down to a question of molecular
replication. Modern DNA molecules - the stuff of genes -
encode information about other molecules, including enzymes
that enable DNA to replicate, mutate, and evolve as conditions
change. But how did DNA - or perhaps RNA - replicate before
there were enzymes?
Several research groups already have mimicked many of the
necessary steps for molecular evolution (SN 8/7/93, p.91) in
their attempt to re-create conditions leading to the origin of
life. But in their experiments they make new copies of these
molecules artificially, with enzymes helping.
Now, two groups have tricked small pieces of DNA into
making copies by themselves. without enzymatic assistance.
Both teams report their results in the May 19 NATURE.
As a result of this work, "We are a step closer to understand-
ing possible pathways to life" comments James Ferris of
Rensselaer Polytechnic Institute in Troy. N.Y.
DNA and RNA are made up of long chains of nucleotides. In
cells, each link in the chain readily pairs off with its comple-
ment: purines attach to pyrimidines and vice versa.
These connections give rise to DNA's typical structure - a
double-stranded helix - which enzymes help split apart during
cell division. The newly created single strands then act as
templates. Each nucleotide seeks out a new partner. and these
partners align to form a complementary strand, thereby
creating two new double helices.
In test tubes, single purine nucleotides readily assemble on a
pyrimidine template. but the reverse doesn't occur, so replica-
tion comes to a halt with mixed templates. Also, even when
scientists could get molecules to replicate, those molecules
could not make copies of their complements.
However, using DNA fragments with three nucleotides
overcomes this obstacle, leading to the formation of comple-
ments on an ongoing basis. says Giinther von Kiedrowslci from
Albert-Ludwigs University in Freiburg, Germany.
For their experiments, von Kiedrowslci and a colleague put
nucleotide threesomes inta a solution that also contained a six-
nucleotide strand. The matching threesomes then lined up to
make a complementary six-nucleotide strand. This strand. too,
began serving as a template for new strands.
Von Kiedrowski thinks that life's earliest molecules arose
when small DNA fragments came together and served as
templates for longer ones. Such fragments could have formed
on clay substrates, adds Ferris.
Bigger nucleotide fragments also work, report Tianhu Li and
Kyriacou C. Nicolaou, chemists at the Scripps Research InJti-
tute in La Jolla. Calif. They started with a palindromic sequesce
of 24 nucleotides: The order of purines and pyrimidines reads
the same from either end of the strand.
In a slightly acidic solution. a double-stranded DNA fragment
attracted two shorter, 12-nucleotide fragments. which assem-
bled into a third 24-nucleotide strand upon the addition of a
chemical reagent. the scientists report. Mal(ing the test-tube
Solution less acidic or adding more of the 12-nucleotide
fragments causes that third strand to separate from the original
double strand and to act as a template for a second strand
complementary to itself, they add.
"We're not saying that we've created life" says Nicolaou. "but
this is perhaps the first example that molecules can replicate
themselves without the help of any enzymes."
Living systems expand exponentially: Two DNA strands
beget four, which beget eight, then 16, then 32, and so on.
Chemical systems increase incrementally, from one to two to
three copies and so on. These new processes yield molecules at
an in-between rate, say the scientists.
(Copyright (c) 1994 by Science News)
E-Mail Fredric L. Rice / The Skeptic Tank